1. Field of the Invention
The present invention relates to an apparatus for polishing an end surface of a workpiece, and, more particularly, to a holder for supporting a polished end surface of an optical fiber.
2. Description of Related Art
The optical fiber communication is an indispensable communication tool at present and it will be in the future. The main structure of an optical fiber connector used in the field of communication is constituted by an optical fiber 11 passing through a ferrule 12 and then being adhered to the ferrule 12 with an adhesive. The ferrule 12 can be made of plastic, glass or ceramics. The ferrule 12 has a spherical end surface 121 which projects outward and is pressed by an elastic polishing surface and worked with a process which includes coarse grinding, fine grinding and polishing. The finished spherical surface 121 must be worked as a flawless curved surface. An optical axis of the spherical surface 121 can be either parallel to a central line of the optical fiber 11 or inclining a small angle with the central line.
A conventional method for polishing an end surface of the optical fiber is to keep the end surface stationary and to rotate and revolve the polishing surface in the process of polishing the end surface. It can be known from a mathematical analysis disclosed in Taiwan Patent No. 485863 entitled as “Polishing apparatus for optical fiber end surface” that if the movement of the polishing surface is rotation plus revolution, the end surface of the optical fiber is merely arranged on the circumference of the holder, and if there is only revolution without rotation, the wearing degree of every spot on the whole end surface is the same. The end surfaces of a plurality of optical fibers can be equally arranged on the holder evenly. Besides, the reference mentioned above also discloses that if the polishing surface is fabricated to be strip-shaped, one side of the strip-shaped polishing surface is provided with coarse polishing particles and another side of the strip-shaped polishing surface is provided with fine polishing particles. FIG. 2 shows that several holders 2, 3 and 4 are on a strip-shaped polishing surface 5 with a right side 6 of the polishing surface being provided with coarse particles and a left side 7 of the polishing surface being provided with fine particles. The pressure between the end surface of the respective optical fiber of the holders and the polishing surface 5 can be uniform if an arrangement is performed properly. The holders 2, 3 and 4 are capable of sliding on the polishing surface 5 from the side with coarse particles toward the side with fine particles and hence, the process including the coarse grinding, the fine grinding and the polishing can be done sequentially. Thus, the problem of the pressure between the respective end surface and the polishing surface being not uniform can be solved, and, theoretically, the end surfaces of a great number of optical fibers can be polished simultaneously.
Basically, an uneven pressure is originated from the following reasons:    1. The supporting point of the holder is not on a plane of application of force generated from a friction between the end surface of the optical fiber and the polishing surface so that a torque is yielded to slant the holder so as to generate the uneven pressure.    2. The polishing surface not parallel to the holder may yield the uneven pressure too when the holder is fixedly clipped.
Please refer to FIGS. 3 and 4. The conventional apparatus for polishing the end surface of the optical fiber comprises a polishing surface 20, ferrule 22, a holder 23 and a pressing rod 24. An end surface 251, 252 of the respective optical fiber 25 is attached to the ferrule 22. When there is a relative movement between the end surface 251, 252 and the polishing surface 21, the holder 23 is subject to a transverse force at an end face thereof. But, the supporting point is at one of contact points 31, 32 right at this time such that the holder 23 is subject to a counterclockwise torque to slant the holder 23 as shown in FIG. 4 to cause the end surface 251 to be lower than the end surface 252, i.e., it causes the pressure acting on the end surface 251 is greater than on the end surface 252.
For solving the aforementioned problem, U.S. Pat. No. 5,216,846 discloses a spacer for maintaining a constant distance between the workpiece and the polishing surface. Because the polishing surface is elastic, maintaining the constant distance means maintaining a uniform pressure. In such a manner, part of the uneven pressure is borne by the spacer to overcome the preceding problem to some extent, but, the torque is still there because the original structure is kept without change such that the unfavorable factor resulting in the uneven pressure is still there. Moreover, U.S. Pat. No. 6,039,630 discloses a pressure sensor is utilized to measure the pressure instantly and compensate the unevenness of pressure by means of a method with an electronically controlling spring.
Furthermore, U.S. Pat. No. 6,077,154 and U.S. Pat. No. 5,351,445 respectively teach a design for clipping and fixing an end surface of an optical fiber as shown in FIG. 5 in which holder 41 is provided with a plurality of optical fiber fixtures 42 for clipping optical fibers 43. The holder 41 is fixed by the fixtures 42 on the upper side of the polishing surface 45. Because the holder 41 is clipped and fixed, the problem of slanting generated from the torque basically does not happen. But, there is still another problem which has to be solved; because the holder 41 is clipped and fixed, and the direction to which the polishing surface 45 faces is also fixed, the holder 41 and the polishing surface 45 are not able to be accurately parallel to each other. In order to overcome the deficiency, the common practice is to adjust the fixtures 42 angularly for fixing the holder 41. But, this is not a natural contact such that the holder 41 is always a little unparallel to the polishing surface 45. Besides, the pressure between the end surface of the optical fiber 43 and the polishing surface is produced by a force exerting upwards from the bottom, and it is not only too complicated but also not suitable for automation. Furthermore, if the polishing surface is strip as shown in FIG. 2 for being slid with several holders simultaneously, it is then difficult for the end surface on each holder to keep the uniform pressure.
Two principles for end surfaces of optical fibers disposed at different positions on a holder capable of being subjected to a uniform pressure are described in the following:    1. The holder shouldn't be clipped and fixed, and must be naturally contact with the polishing surface in case of the polishing surface is hard and inelastic. That is, the holder oscillates up and down with oscillation of the polishing surface, and the average pressure acting on the end surface at different positions on the holder keeps uniform when the holder oscillates up and down. The holder and the polishing surface must be kept fairly parallel to each other, and in the meantime, the polishing surface is capable of being pressed if the polishing surface is elastic.    2. A plane formed by the end surface in the process of polishing is a plane of application of force that is a plane constituted by a force exerted to the holder by the polishing surface. It is necessary to provide one or several supporting points for the holders because, in principle, the holder does not move or rotate with the polishing surface in the process of polishing. In order to obtain the uniform pressure, these supporting points must be on the plane of application of force to keep the holder from moving or rotating with the polishing surface.
Please refer to FIG. 6. U.S. Pat. No. 7,063,062 discloses that a plurality of symmetrical accepting grooves 61 for receiving counterweights 62 are disposed on the periphery of a main body 60 of a holder; in this way, the main body 60 allows the end surfaces of a plurality of optical fibers 63 to have down pressing forces exerting on a polishing surface 64 such that the main body 60 contacts naturally with the polishing surface 64. A plurality of symmetrical fixing rods 65 are disposed at the sides of the main body 60, a contact portion 651 is disposed on the bottom of each fixing rod 65 and positioned in a fixing groove 661 of each framework 66. A weephole 662 for draining fluid dropping into the accepting grooves 661 during polishing. A contact surface between the contact portion 651 and the fixing groove 661 is the same as a contact surface between the bottoms of the optical fibers 63 and the polishing surface 64 to allow the supporting points of the main body 60 to be on a plane of application of force generated by the bottoms of the optical fibers 63 and the polishing surface 64. Thus, a torque is not yielded to slant the main body 60 and a phenomenon of generating the uneven pressure can be avoided.
The symmetrical fixing rods 65 of the holder shown in FIG. 6 fix the main body 60 such that the main body 60 is incapable of rotating. Only a portion of the polishing surface 64, which contacts with the end surface of the optical fiber, is used instead of all polishing points of the polishing surface 64 being used uniformly. Besides, bottom ends of the four fixing rods 65 of the main body 60 must be kept at the same plane, and it is a very difficult manufacturing process.
U.S. patent application Ser. No. 11/812,967 discloses a holder for supporting an end surface of an optical fiber during polishing. A main body and a framework of the holder are respectively provided with corresponding fixing portions, at least one of the two fixing portions is provided with at least one set of upper and lower contact portions, the upper contact portion and the lower contact portion on each set are respectively disposed at the upper side and the lower side of a contact plane between the end surface of the workpiece and a polishing surface. Restriction to mutual contacts between the upper contact portion, the lower contact portion and another fixing portion is utilized for the main body being capable of maintaining parallel to the polishing surface so as to enhance the polishing speed and quality.